The mineral coal is a complex mineral of widely varying composition and structure, dependent upon the location and conditions under which it was formed in nature. In general, coal is classified or ranked according to its content of volatile matter which can range from around 50% or more for lignite or cannel coal to about 20-30% for a middle rank bituminous, gas or coking coal to 10% or less for a high ranking bituminous coal or anthracite, the remainder being constituted by nonvolatile or fixed carbon together with minor amounts up to about 8% or so of each of ash and moisture.
It has long been known to subject coal to pyrolytic destructive distillation to drive off the volatile content and leave the nonvolatile matter in a solid form known as coke having valuable properties as a fuel in the production of iron and steel and in the production of gases for heating and illumination. As originally practiced, the volatile matter was allowed to escape as waste into the atmosphere, but it was soon realized that the volatile matter was valuable in itself by virtue of the inclusion therein of a large number of organic chemicals having valuable utility in industry in themselves or as intermediates for the formation of technologically important derivatives. There are now known to be contained in coal tar extracted from coal nearly 300 different organic chemical compounds including benzene and its alkylated and partially or totally hydrogenated derivatives, styrene, naphthalene and anthracene and their derivatives together with numerous other carbocyclic and heterocyclic hydrocarbons, particularly those based on fused ring systems.
The temperature and other conditions of the pyrolytic decomposition or carbonization of the coal can vary considerably in order to tailor the output of the process to exaggerate the formation of certain particularly desirable compounds. Where the process conditions are selected as to be especially severe, it is usually referred to as a gasification process, the object of these conditions being to magnify the gaseous content of the reaction as greatly as possible. These vapor phase products can be condensed to produce oil fractions useful directly or by intermediate conversion, as by catalytic reforming and/or cracking as diesel oil and gasoline for internal combustion engines. Direct hydrocarbonization gasification processes subject the coal to hydrogen gas under high pressure in the order of about 50-100 atmospheres and are consequently expensive and difficult to practice, although such processes have become increasingly the object of concentrated research as an alternative source of IC engine fuel to natural petroleum.
In another type of coal gasification, the coal is heated in the presence of air, steam, oxygen or a combination thereof, to produce by reaction of the carbon in the coal, fuel gases of varying proportions of carbon monoxide, carbon dioxide, hydrogen and occasionally nitrogen for industrial and domestic heating, as a source of hydrogen or for further conversion. In modern processes of this type, the coal is introduced continuously into a moving or fluidized bed reaction zone with the gaseous products being taken off from the top of that zone and unreacted solid residue from its bottom. Because of the interference of substantial amounts of tar, the application of the process is hence limited.
It is also known to subject coal to so-called liquefication processes in which the coal is treated under less severe conditions than utilized for carbonization and gasification, usually at temperatures below about 600.degree. C. at which substantial gas formation is initiated and under pressure. Even at this condition, coal is difficult to dissolve and heavy attention has been directed in research in this field to the identification of solvents capable of dissolving the coal and for the most part the solvents found to be more or less effective have been based on hydrogen-rich or protonic organic liquids, usually derived from the coal itself or as specialized by-products from the distillation and fractionation of petroleum, having a chemical structure adapted to compensate the natural hydrogen deficiency of coal which tends to impede its dissolution. Such processes are frequently carried out under high pressure in a hydrogen atmosphere to make available additional hydrogen atoms for combination with the coal. The following is a list of patents which relate to this kind of coal liquefication process:
2,572,061; PA0 3,375,188; PA0 3,379,638; PA0 3,642,608; PA0 3,705,092; PA0 3,726,785; PA0 3,849,287; PA0 3,852,183; PA0 3,867,275; PA0 3,870,621; PA0 3,956,436; PA0 4,040,941; PA0 4,052,291; PA0 4,052,292; PA0 4,189,373.
Even though a fraction of the reaction products from the liquefication process may be withdrawn and recycled for combination with fresh amounts of coal, these processes are fundamentally independent on the derivation of solvents directly from natural energy materials which might be better used for their usual purposes. In addition, in their modern versions, these processes can be carried out in the presence of finely divided solid catalysts serving to increase the efficiency of the reaction and/or bias the reaction toward the formation of particularly desirable end products, e.g., gasoline and diesel oil, and these catalysts inherently tend to become poisoned in time so as to lose their effectiveness.
Moreover, various recovery techniques are necessary for the separation and purification of the liquefication products, including packed columns and filters and as the liquefication products contain a high content of tar, wax and the like, the separation units are seriously susceptible to clogging which requires cleaning and replacement from time to time.
Finally, in the rare instances in the art where coal has been subjected to simple extraction, e.g. U.S. Pat. No. 2,242,822, preliminary oxidation of the coal has been indispensable to convert it into a form susceptible to dissolution in furfural and furane derivatives employed as solvents.